1. Field of the invention
The present invention relates to an FM-CW radar apparatus using a frequency-modulated continuous wave as a transmitted wave and, more particularly, to the FM-CW radar apparatus that can accomplish a beam scan by digital beamforming (DBF).
2. Related Background Art
An example of the DBF radar apparatus is one described in Japanese Patent Application Laid-Open No. 6-88869. In this conventional radar apparatus, an RF amplifier, a mixer, a filter, and an A/D converter are connected to each of antenna elements constituting an array antenna, digital signals outputted from the respective A/D converters are read into a digital beamforming processor, and the digital beamforming processor carries out the digital beamforming operation, based thereon.
Generally speaking, the radar apparatus uses high-frequency electromagnetic waves such as microwaves or millimeter waves, but analog devices (such as the RF amplifiers and mixers) operating at such high frequencies are very expensive.
The conventional radar apparatus described above necessitates a number of analog devices, because such analog devices are given to each of the antenna elements. Therefore, the production cost becomes inevitably high. Particularly, it is conceivable as a means for improving the performance that the number of antenna elements is increased, but the increase of the antenna elements also increases the number of high-frequency analog devices attendant thereon, thereby increasing the cost. It was thus difficult to increase the number of antenna elements. In addition, the increase of analog devices also increases the scale of the radar apparatus.
An object of the present invention is to provide an FM-CW radar apparatus including the minimum number of analog devices, regardless of the number of antenna elements.
The FM-CW radar apparatus of the present invention comprises a transmitter section, a receiver section, and a signal processing section. The transmitter section transmits a frequency-modulated continuous wave as a transmitted wave. The receiver section receives radio waves resulting from re-radiation from a target, as received waves, through a receiving antenna comprised of an array of antenna elements, generates a beat signal which is a difference between the transmitted wave and a received wave in each of channels of the respective antenna elements, and converts this beat signal to a digital beat signal by A/D conversion. The signal processing section executes the digital beamforming operation with the digital beat signals to detect the target from the result thereof.
The receiver section comprises switch means for selectively connecting either one of the antenna elements to a circuit for generating a beat signal and this switch means connects only part of the antenna elements to the beat signal generating circuit in one period of repetition periods of frequency modulation.
With the FM-CW radar apparatus of the present invention constructed as described above, because the switch means selectively connects either one of the antenna elements in sequence to the circuit for generating the beat signal, the received waves through the respective antenna elements can be supplied in time division to the circuit for generating the beat signal. There is thus no need to prepare the high-frequency devices, such as the mixer circuit etc. for downconversion of the received wave, for each of the antenna elements, and only one set will suffice.
In addition, since the switch means connects less than all of the antenna elements to the beat signal generating circuit in one of repetition periods of the frequency modulation, the switching frequency may be lower than that in the case wherein all the antenna elements are connected to the beat signal generating circuit in one of repetition periods of the frequency modulation.
Since the A/D conversion is considered to be carried out based on sampling of a beat signal every switching of connection, an A/D conversion rate may also be reduced with decrease of the switching frequency.
It is desirable that the switch means should select one antenna element as a reference antenna element in each of repetition periods of the frequency modulation and that the signal processing section should correct phases of waves received by the antenna elements other than the reference antenna element, based on a phase difference of a wave received by the reference antenna element in each of repetition periods of the frequency modulation.
Between different periodic intervals of repetition periods of the frequency modulation, the distance to the target may vary during a time difference. In that case, there will occur a difference between phases of received waves. Namely, sufficient simultaneity of reception is not assured for every antenna element. In contrast with it, since the apparatus is constructed to detect a phase difference of a signal from the reference antenna element in each period and to correct phases of signals of the antenna elements other than the reference antenna element, based on this phase difference, the apparatus may perform the DBF synthesis almost equivalent to that in the case wherein the signals from all the antenna elements are read in one period.